HISTORY OP PHYSICAL ASTRONOMY.

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siderable accuracy several periods relating to the motion of the moon. Theearliest astronomical observation recorded as having been made by theGreeks previous to the establishment of the Alexandrian School , is a deter-mination of the summer solstice by Meton , in the year 430 a.c. Theinstrument, termed a heliometer, which was used by Meton on this oc-casion, was, in all probability, no other than a modification of the gnomon.The date of this solstice has been chosen for the epoch of the Metoniccycle of nineteen years, which is employed in regulating the occurrence ofreligious festivals.

A new era commenced in the history of astronomical observation whenAlexandria became the capital of the civilised world. Under the liberalpatronage of the Ptolemies a magnificent building was erected, in whichwere deposited circular instruments for determining the positions of theheavenly bodies, and every facility was given to astronomers for prosecutinga continuous series of observations. Timocliaris and Aristillus are theearliest individuals mentioned in connexion with this school. These astro-nomers appear to have flourished about the year 300 a.c. Ptolemy citesseveral of their observations in the Syntaxis . Among these are the de-clinations of a few of the principal stars. It is evident that such resultscould not be established without a knowledge of the position of theequator in the celestial sphere. It does not appear that the earlier astro-nomers of the Alexandrian School were acquainted with any method fordetermining the right ascensions of the stars. At any rate Ptolemy doesnot cite any observations of this kind as having been made by them. Indeed,there are no grounds for supposing that they knew the exact position ofthe equinoctial colure upon which the right ascension of a star depends.It is a remarkable fact, however, that Hipparchus was enabled by meansof certain eclipses of the moon, observed by Timocbaris, to determine theplace occupied by the equinox among the stars in the days of that astro-nomer. Thus, Ptolemy states that Hipparchus having compared severaleclipses of the moon, observed very accurately by himself, with other moreancient eclipses, observed by Timocharis, found that while the bright star,Spica Virginis, preceded the Autumnal equinox 8° in the time of the last-mentioned astronomer, it preceded the same equinox only 0° in his owntime *. No account whatever is given of the process by which theseinteresting results were arrived at.

* Syntaxis, lib. vii., cap. ii. With respect to the mode in which Hipparchus determinedthe place of the equinox by means of his own observations of eclipses of the moon, it wasdoubtless the same as that which will be presently alluded to in the text. But it is diffi-cult to imagine by what process he was enabled to deduce a similar result from the obser-vations of Timocharis, since there are no grounds for supposing that the latter astronomerpossessed an intrument for measuring the difference of the right ascensions of twocelestial bodies. It is probable that if a bright star happened to be very near the moon atthe time of an eclipse, its position in the direction of the zodiac with respect to that bodywas roughly determined with the instrument employed in measuring the declinations ofthe stars, and the result recorded as part of the observation of the eclipse. It is notdifficult to conceive that an instrument composed of a circle fixed in the planeof the equator, and another circle of equal magnitude moveable about its poles,might serve for the approximate measurement of the difference of the right ascensions oftwo celestial bodies situate very near the equator. The earlier astronomers of the Alex-andrian School , in all probability, did not contemplate any definite object in noting- the po-sitions of the stars in the vicinity of the moon during the eclipses of that body. Indeed,it would have been impossible to deduce the place of the equinox among the stars fromsuch results, even if they had been accurately determined with the astrolabe; for theplace of the moon, with respect to the equinox, could not have been ascertained withoutsolar tables; but there is not the smallest reason to suppose that astronomers were ac-

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